Indexing fix for gpt_bigcode (#22737)

Fix indexing

src/transformers/models/gpt_bigcode/modeling_gpt_bigcode.py

@@ -538,7 +538,7 @@ class GPTBigCodeModel(GPTBigCodePreTrainedModel):
     def forward(
         self,
         input_ids: Optional[torch.Tensor] = None,
-        past_key_values: Optional[Union[List[torch.Tensor], int]] = None,
+        past_key_values: Optional[List[torch.Tensor]] = None,
         attention_mask: Optional[torch.Tensor] = None,
         token_type_ids: Optional[torch.Tensor] = None,
         position_ids: Optional[torch.Tensor] = None,
@@ -584,7 +584,7 @@ class GPTBigCodeModel(GPTBigCodePreTrainedModel):
             past_length = 0
             past_key_values = tuple([None] * len(self.h))
         else:
-            past_length = past_key_values[0][0].size(-2)
+            past_length = past_key_values[0].size(-2)
 
         if attention_mask is not None and len(attention_mask.shape) == 2 and position_ids is None:
             # create position_ids on the fly for batch generation